The recovery of carbon dioxide from gaseous mixtures is propelled by multiple factors including the industrial carbon dioxide market, enhanced oil recovery (EOR), and greenhouse gas emissions reduction. In the United States, carbon dioxide emissions have not been federally regulated to date. In 2007, the U.S. Supreme Court ruled that the Environment Protection Agency (EPA) has the authority to regulate carbon dioxide emissions. Additionally, on Jan. 26, 2009, United States President Obama signed a Presidential Memorandum asking the EPA to review its previous denial of California's request for a waiver of a statutory prohibition on State adoption or enforcement of emission standards. California had sought the waiver to adopt limitations on greenhouse gas emissions, including carbon dioxide, from motor vehicles. As a result, carbon dioxide recovery has recently become more important to many corporations doing business in the United States.
Many have proposed methods of recovering carbon dioxide from gaseous mixtures. Some examples include the use of pressure or temperature swing adsorption (PSA or TSA) alone or in combination with membranes. U.S. Pat. No. 6,309,445 to Gittelman et al. teaches the removal of carbon dioxide from air via PSA or TSA. U.S. Pat. No. 5,411,721 to Doshi et al. teaches removal of carbon dioxide from a natural gas feedstream using the combination of a membrane and PSA. However, PSA tends to be energy intensive due to the requirement to compress the entire feed stream, rendering the process expensive and unsuitable for many applications.
Vacuum Swing Adsorption (VSA) is another method used to separate gases. Like PSA, VSA uses adsorbents to segregate certain gases in a gaseous mixture under minimal pressure according to each gas' molecular characteristics and affinity for the adsorbents. These adsorbents preferentially adsorb the target gas species at near ambient pressure. The process then utilizes a vacuum to produce a target species-rich stream and to regenerate the adsorbent material.
VSA is sometimes described as a subset of the PSA category. However, VSA differs from PSA in that PSA uses a pressurized gas feed in the separation process and typically vents to atmospheric pressures while VSA draws the gas feed through the separation process with a vacuum. Additionally, VSA operates at near-ambient pressures. Hybrid Vacuum Pressure Swing Adsorption (VPSA) systems also exist in which the gas feed is pressurized prior to the separation process, as in the PSA process, while a vacuum is used to produce the target species-rich stream and to regenerate the adsorbent material, as in the VSA process.
U.S. Pat. App. Pub. Nos. 2007/0227353, 2007/0232706, and 2008/0072752, all assigned to Praxair, Inc., utilize a VSA process to remove carbon dioxide from a synthesis gas stream formed within a steam methane reformer. However, steam methane reformers produce a high pressure emission source of about 200 to about 500 psia, or approximately 13 bar to 35 bar. Paul Webley et al. have used a VSA process to recover carbon dioxide from exhaust gases (see abstract from the American Institute of Chemical Engineer's 2005 Annual Meeting #439b). However, purity of the recovered carbon dioxide was limited to a maximum of 90% and therefore was not suitable for many applications. Low cost methods to recover and purify carbon dioxide from low pressure gaseous mixtures having a low concentration thereof are lacking.